A conventional telescopic centerpost of a multiple-fold umbrella as shown in FIG. 1 comprises an arcuate spring plate A having its upper catch C1 supporting a slider or runner S of umbrella spokes and having its lower catch C2 operatively coupling an upper tube T1 and a lower tube T2 telescopically connected together. In order to retract so many telescopic tubes within the slider S, the aperture S1 should be large enough for retracting the tubes of the centerpost. However, a width of the upper catch C1 can not be made too large since the catch C1 must be limitedly retracted into the tube T1 during a retraction of the centerpost, thereby causing an unstable supporting of the slider on the upper catch C1 of the spring plate A. Meanwhile, the catch C1 only protrudes leftwardly to support a left portion of the slider so that a right portion of the slider will be collapsed inclinedly, causing a local stress or deformation of the spokes and canopy of the umbrella.
Tilmann Schultes et al. disclosed a shortenable umbrella frame having a telescopic stick in their U.K. patent application GB No. 2179855 A, in which a plug 6 having annular shoulder 6A is formed in the stick for firmly coupling an upper tube 2c and a lower tube 2b as retained by a catch 5 as shown in their FIG. 1. When extending the telescopic tubes 2b, 2c, the tubes are frictionally engaged at plug 6 and shoulder 6a for a stable extension of the stick 2, which however requires a bigger force to separate the two tubes 2c, 2b when retracting the stick 2. The catch 3 with a single ball 3a may only exert a very limited spring force for fastening the telescopic tubes, which may then be easily loosened during a vibrational movement of the umbrella.
The present inventor has found the drawbacks of the telescopic stick or centerpost of a conventional umbrella, and therefore invented the present centerpost with stable structure and convenient operation.